(The following account is from Galatians 4:21-31 KJV).
Abraham, a patriarch of the Old Testament, had two sons. God had promised Abraham and his barren wife Sarah that He would bless them with a son in their old age. Due to a lack of faith, Sarah gave Abraham her maidservant, Hagar, to whom Ishmael was born. The son, promised by God, was later born to Sarah when she was ninety years old and Abraham was one hundred. Ishmael was born after the flesh to a bondswoman who was not free while Isaac was born to a free woman by the promise of God.
This is a beautiful allegory of the two covenants by which God has governed His people throughout the dispensation of time—the law system and the gospel. Hagar and Ishmael were as Mount Sinai where God instituted the first covenant and gave the law to Moses. The children of Israel were slaves to the rites, ceremonies, and commandments of the law. This first covenant, the law system, was after the flesh just as Ishmael was born after the flesh. Hagar was not free and could only deliver her children into conditions of slavery. Likewise, those born under the Mosaic law were in bondage to a system in which they were never able to fully measure to its obligations.
Isaac, born of faith, a child of promise by the divine intervention of God, represented the new covenant that God made with mankind through His Son Jesus Christ. Isaac’s position superseded that of his brother Ishmael. Isaac was a type of Christ, who was offered on the altar of sacrifice in obedience to the will of the Father. God’s covenant with Abraham was fulfilled through Isaac from which came a mighty nation, God’s chosen people.
A parallel is drawn between Sinai and Jerusalem. Jerusalem, while under the bondage of the Mosaic law and the political governance of the Roman Empire, was also the location of the temple where God was worshiped. It was the center of the new Christian faith. Spiritual Jerusalem, in contrast to Sinai, represents a place where all her citizens are born under the new covenant and are free.
“Jerusalem which is above is free, which is the mother of us all” (26). True Christians, living under the covenant of the gospel, are free from the bondage of the law with all its rites and customs. They are free from sin and condemnation. They are born not of the slave but of the free woman.
As Hagar and her son were cast out, so the apostle Paul instructed the church to cast out the old covenant. “The son of the bondwoman shall not be heir with the son of the freewoman” (30). Isaac was the son of promise, he was of faith, he was the fulfillment of God’s plan for the Jewish nation, and he was the heir. The Mosaic law is to be cast out in favor of the new covenant. Christians, followers of Jesus Christ, are the children of promise.
Some people cleave to the letter of the Mosaic law or possess an old covenant mentality while living under the gospel. This yields itself to bondage. Cast out the old covenant and enjoy the liberty and freedom of the gospel covenant, for “we are not children of the bondwoman, but of the free” (31).